1. Field of the Invention
The present invention relates to a motor control device for driving and controlling a motor.
2. Description of Related Art
In order to perform a vector control of a motor by supplying three-phase AC power to the motor, it is necessary to detect phase currents of two phases (e.g., the U-phase current and the V-phase current) among three phases including the U-phase, the V-phase and the W-phase. Although two current sensors (current transformers or the like) are usually used for detecting the phase currents of two phases, the use of two current sensors causes an increase of cost of the entire system equipped with the motor.
For this reason, a conventional method is proposed, in which bus current (DC current) between an inverter and a DC power supply is sensed by a single current sensor, and the phase currents of two phases are detected from the sensed bus current. This method is also called a single shunt current detecting method.
FIG. 41 shows a general block diagram of a conventional motor driving system in which the single shunt current detecting method is adopted. An inverter (PWM inverter) 902 is equipped with half bridge circuits for three phases, each of which includes an upper arm and a lower arm, and it converts a DC voltage from a DC power supply 904 into a three-phase AC voltages by switching the individual arms in accordance with three-phase specified voltage values given by a controller 903. The three-phase AC voltage is supplied to a three-phase permanent-magnet synchronous motor 901, so that the motor 901 is driven and controlled.
A line connecting the individual lower arms in the inverter 902 with the DC power supply 904 is called a bus line ML. A current sensor 905 transmits a signal indicating the bus current that flows in the bus line ML to the controller 903. The controller 903 performs sampling of an output signal of the current sensor 905 at appropriate timing so as to detect a phase current of the phase in which a voltage level becomes a maximum value (maximum phase) and phase current of a phase in which a voltage level becomes a minimum value (minimum phase), i.e., the phase currents of two phases.
If voltage levels of phases are separated from each other sufficiently, the phase currents of two phases can be detected by the process described above. However, if the maximum phase of voltage and an intermediate phase become close to each other, or if the minimum phase of voltage and the intermediate phase become close to each other, it is difficult to detect the phase currents of two phases. Note that the description of the single shunt current detecting method including the description of the reason why it becomes difficult to detect the phase currents of two phases will appear later with reference to FIGS. 3, 4 and 5A-5D.
Considering this, a method is proposed in which if the phase currents of two phases cannot be detected by the single shunt current detecting method in a certain period, pulse widths of PWM signals with respect to arms in the inverter are corrected based on three-phase gate signals.
An example of usual correction of the specified voltage value (pulse width) that corresponds to the above-mentioned correction is shown in FIG. 42. In FIG. 42, the horizontal axis indicates time, and reference numerals 920u, 920v and 920w denote voltage levels of the U-phase, the V-phase and the W-phase, respectively. Since a voltage level of each phase follows the specified voltage value (pulse widths) for each phase, they are considered to be equivalent. As shown in FIG. 42, the specified voltage value (pulse width) of each phase is corrected so that “a maximum phase and an intermediate phase” as well as “a minimum phase and the intermediate phase” of the voltage do not approach each other closer than a predetermined distance. Thus, voltages of individual phases do not become close to each other to the extent that the phase currents of two phases cannot be detected, and the phase currents of two phases can be detected stably.
On the other hand, as a method for estimating a rotor position without using a position sensor, there is proposed a method of applying a high frequency voltage. It is known that the rotor position estimating method based on the application of a high frequency voltage can perform a good estimation at a standstill or at a low rotation speed because the method utilizes salient magnetic poles of the motor.
It is possible to realize low cost and stable estimation at a standstill or the like by combining the single shunt current detecting method with the rotor position estimating method based on the application of a high frequency voltage, but a method for realizing the combination is not proposed yet. Therefore, as a matter of course, there is no technique about items to be considered when both the methods are combined.